Safely and efficiently transporting personnel to oil platforms in the open ocean is a formidable challenge. In particular, wave heights of two to three meters and thirty-knot winds are not uncommon. In these conditions, transfer vessels experience pronounced heaving, pitching, and rolling motions, especially when they are at zero forward speed.
Traditionally, crews have been transferred to an oil rig via a crane-and-basket method or using a basket that is deployed from a helicopter. In the former method, personnel being transferred from a vessel step into or hang on to a basket that is suspended from a rig-mounted crane. The crane then hoists the basket and swings it over to the rig. In the latter technique, personnel are lowered from a helicopter on to the rig via a basket.
Used for the decades, both of these personnel-transfer methods involve certain risks. The usual accidents include lateral impacts, falling, hard landings, and water immersion.
Furthermore, the crane-and-basket method relies on the availability of the platform crane operator. A delay caused by the non-availability of a crane operator when needed results in down-time costs as well as an increase in the incidence of seasickness due to personnel spending an extended period time on a stationary but heaving/pitching/rolling transport vessel.
More recently, a gangway technique has been used wherein the free end of a ramp that is disposed on the oil rig is rotated toward and landed on a crew-transfer vessel. This technique is only suitable for use in relatively low sea states (e.g., sea state 2, etc.) since relatively higher sea states can cause substantial movement of the ramp. Such movement can present a safety risk to personnel that are using the ramp to transfer to an oil rig.